The present invention relates to an electronic flash capable of automatic flash duration control and more particularly an electronic flash having a main flash capacitor, a trigger circuit, a flash lamp which is triggered by a trigger voltage from the trigger circuit and converts the electrical energy stored in the main flash capacitor into a flash of light, a light reception circuit for integrating the amount of light reflected back from a subject exposed by the flash of light and a flash duration control circuit or a flash interruption circuit for interrupting the flashing of the flash lamp when the amount of reflected light integrated by the light reception circuit reaches a predetermined level.
When the electronic flash of the type described receives light other than reflected back from a subject exposed by a flash of light prior to the time the flash lamp is flashed, the erratic integration of light occurs in the light reception circuit.
In order to overcome this problem, there has been devised and demonstrated an electronic flash of the type described in U.S. Pat. No. 3,818,266. In this electronic flash, in addition to the main flash capacitor an additional capacitor is provided which is discharged through the flash lamp, a resistor and a constant voltage element such as a zener diode simultaneously when the flash lamp is flashed, and a constant voltage across the constant voltage element is supplied to the light reception circuit to actuate the same.
The voltage across the constant voltage element is supplied to the light reception circuit before the flash lamp is flashed so that the erratic operation of the light reception circuit due to light other than light reflected back from the subject exposed by a flash of light may be avoided.
However, when the flash lamp is flashed, the trigger voltage has not been sufficiently damped yet so that the erratic operation results. The trigger voltage is a high frequency voltage at hundreds of thousands of hertz to a few megahertz so that when the high-frequency voltage is not sufficiently damped, the light reception circuit integrates light other than light reflected back from the subject exposed by a flash of light and consequently the accurate integration of light cannot be attained.